1. Field of the Invention
The present invention relates to a thermal analyzer for measuring how the physical or chemical properties of a sample as a function of temperature, and particularly to a differential scanning calorimeter for measuring and analyzing a endothermic and exothermic phenomena of a sample by detecting a differential heat flow between sample and reference material under the condition of constant rate temperature change. Heat flow that a sample additionally generates or absorbs compared to a reference material when temperature is changed at a constant rate.
2. Description of the Related Art
A differential scanning calorimeter is a device for differentially detecting and analyzing heat flow that a sample additionally generates or absorbs compared to a reference material, when the temperature of both the sample and the reference material is changed at a constant rate with the sample and the reference material (alumina or the like is normally used for a thermally stable reference material) arranged symmetrically.
When the temperature of the sample rises at a constant rate, heat absorption by the sample increases as the heat capacity of the sample becomes large. Specifically, an absolute value of differential heat flow signal becomes large. At this time, from the fact that the absolute value of the differential heat flow signal is proportional to change in thermal capacity between the sample and a reference and to rate of temperature rise, the heat capacity of the sample can be known from original differential heat flow signal based on an already known rate of temperature rise and heat capacity of the reference.
On the other hand, when a sample is fused, heat absorption by the sample temporarily becomes large, and if the differential heat flow signal is recorded over time and plotted on a graph it will appear as an endothermic peak. Also, if the same recording method is used, then the differential heat flow signal will describe an exothermic peak if crystallization occurs in the sample. The surface area of these endothermic and exothermic peaks described on the time axis set up so that a unit time corresponds to a constant length is proportional to an amount of heat released or absorbed when the sample is in transition, which means that if an already known heat of transition is measured in advance and a signal value calibrated, it is possible to easily obtain the heat of transition of the sample from a differential heat flow signal. Differential thermal calorimeters are widely used in the analysis of various materials in order to obtain a differential heat flow signal having the above described useful characteristics.
Conventional differential scanning calorimeters are generally divided into the following two types.
First of all there are the power compensation type, comprising a combination of two independent calorimeters for a sample and for a reference arranged symmetrically, and respectively provided with a resistance temperature sensor and a heater for heat flux feedback. An average value of the temperatures detected by the two sensors is compared to a temperature output of a temperature program device varied at a constant rate, and the two calorimeters are heated by the heaters for heat flux feedback so that the average value and the temperature output coincide. Also, if there is a difference in temperature output of the two temperature sensors, electrical power of both heaters is immediately adjusted so that the difference returns to zero. At this time, a difference in electrical power supplied to the two heaters every second is recorded as a differential heat flux signal. A power compensation-type differential scanning calorimeter is extremely responsive, and is capable of realizing a thermal compensation time constant of less than two seconds.
There are also heat flux type calorimeters in which temperature sensors for a sample and a reference are fixed inside a heat sink formed of a superior heat conducting material so as to form symmetrical heat flow paths that are equal to each other. The temperature of the heat sink is compared to a temperature output of a temperature program device varied at a constant rate, and feedback control is performed using a heater wound around the heat sink so that the temperature of the heat sink and the output temperature coincide. A temperature difference between the sample and the reference is detected using a differential thermocouple. At this time, it is possible to obtain differential heat flux which is a difference between heat flow to the sample and heat flow to the reference, if the temperature difference between the sample and the reference is divided by the thermal resistance (reciprocal of thermal conductance) between the heat sink and the sample, this is similar to obtaining electrical current by dividing a potential difference by resistance, Specifically, with a heat flux type differential scanning calorimeter, an output of a differential thermocouple, representing a temperature difference between the sample and the reference, is appropriately amplified, and output and stored as a differential heat flow signal.
A heat flux type differential scanning calorimeter has excellent base line stability, but ordinarily has a thermal compensation time constant of over 3 seconds, which means there are problems such as the fact that a heat flow signal peak is not so sharp, and separation between a plurality of peaks becomes poor. With an power compensation type, it is possible to realize a thermal compensation time constant of less than two seconds but with respect to base line characteristics, it has been difficult to get stability as good as that of the heat flux type differential scanning calorimeter. The main reason for this is that there is a large temperature difference between the power compensation type sensor and the surrounding material during measurement, which means that there is constantly a comparatively large amount of heat leakage from the sensor to the outside, constituting the main cause of drift in the base line.
A method of combining a power compensation type detector and a heat sink formed of a material having a good heat conductivity, which is the main feature of the heat flux type calorimeter, has been disclosed, with the intention of solving the drawbacks of the above described two types of differential scanning calorimeter. In Japanese Patent Laid open No. Tokkaihei. 11-160261, there is disclosed a differential scanning calorimeter, comprising: a heat sink formed of a thermally good conductor and having a space for accommodating a sample at an inside;
a detector fixed within said heat sink and formed by an insulation substrate formed with symmetric circuit patterns of metal resistors;
a temperature measuring circuit for measuring a temperature of said detector by detecting resistance values of said metal resistors in said detector;
a differential temperature detecting circuit for comparing resistance values of one pair of metal resistance circuit to detect a temperature difference between a sample and a reference placed in said detector;
a program temperature function generator for outputting temperature target values in time;
a heat sink temperature controller for controlling a temperature of said heat sink depending on an output of said program temperature function generator;
a detector temperature controller for controlling a temperature of said detector by controlling current values flowing through said metal resistance circuits in said detector based on a comparison result of an output of said program temperature generator and output of said temperature measuring circuit;
a differential heat compensating circuit for causing a proper current to flow through each of said one pair of metal resistor in said detector such that an output of said differential temperature detecting circuit uninterruptedly returns to zero;
whereby low drift characteristic of a heat flux type and high responsibility of a power compensation type are both obtained.
As disclosed in Japanese Patent Laid-open No. Tokkaihei 11-160261, in a differential scanning calorimeter comprising a temperature detector having circuit patterns using metallic resistors and an insulating substrate having a compensation heater fixed inside a heat sink formed of a material having good heat conductivity, the temperature difference detecting circuit uses a bridge circuit, and detection is carried out by causing current to flow in the metallic resistors of the circuit patterns and detecting resistance values from potential differences of the bridge circuit. Accordingly, it is fundamentally impossible to avoid the fact that there is self-heat generation caused by the electrical current flowing in the metallic resistors themselves of the circuit pattern, being the temperature detector. As well as performing control of the compensation heater with the temperature difference sensor as a trigger, if there is temperature variation caused by self heat generation at the time of temperature detection with an initial trigger, this variation is amplified because of negative feedback of the compensation heater based on this temperature variation, and as a result there is a problem that it is difficult to obtain stable temperature control.
Also, in the differential temperature detecting circuit for detecting temperature difference, if the voltage applied to the bridge circuit is lowered in order to reduce self heat generation in the circuit pattern, a potential difference corresponding to the temperature difference is also reduced, and the electrical sensitivity of the temperature difference detection is lowered. On the other hand, it is necessary to raise the applied voltage in order to increase temperature difference detection sensitivity, and in this case there the dilemma that self heat generation in the circuit patterns increases.
In order to solve the above described problems, a differential scanning calorimeter according to the present invention has both a high response of power compensation type and a baseline stability of heat flux type and is provided with a heat sink made of a good heat conductive material and having a space for housing a sample and sample reference material inside; an insulating substrate fixed to the inside of the heat sink and setting a region for mounting the sample and the reference; first circuit pattern that has been applied so as to form junction of two kinds of alloy or metal at respective regions for mounting the sample and the reference on the insulating substrate; a differential temperature detector for detecting a thermal difference between the sample and the reference mounted on the insulating substrate by detecting a difference in thermoelectromotive force at junctions of sample side region and of the reference side region of the circuit pattern; second circuit patterns formed by means of metallic resistors respectively applied to regions for mounting the sample and the reference on the insulating substrate; a differential heat compensation circuit for allowing respectively appropriate currents of the second circuit patterns formed by means of metallic resistors on the insulating substrate to flow so as to continually return output of the differential thermal detection circuit to zero; a temperature program generator for outputting a target temperature value periodically; and a heat sink temperature controller for controlling temperature of the heat sink in response to output of the temperature program generator.
A sample and a reference are subject to temperature control utilizing thermal conduction from a heat sink controlled in response to a program temperature, through an insulation substrate. A temperature difference between the sample and the reference under temperature change is detected as a thermo electromotive force generated by a Seebeck effect of applied circuit patterns forming junction of two kinds of metal or alloy, and output to a differential heat compensation circuit. The differential heat compensation circuit adjusts the amount of electrical power to the respective metallic resistors applied to mounting regions for the sample and the reference so that a temperature difference immediately returns to zero, and metallic resistors act as heaters for power compensation provided individually close to the sample and the reference.
As a result, differences in absorbed or generated heat compared to the sample and the reference are detected as differences in power supplied to heaters provided individually close to the sample and the reference, and realize a function of a differential scanning calorimeter.